U.S. patent number 9,479,031 [Application Number 13/790,135] was granted by the patent office on 2016-10-25 for tubular linear motor with magnetostrictive sensor.
This patent grant is currently assigned to MTS SENSOR TECHNOLOGIE GMBH & CO. KG. The grantee listed for this patent is MTS Sensor Technologie GmbH & Co. KG. Invention is credited to Andre Beste, Manfred Sapp, Ingo Walter.
United States Patent |
9,479,031 |
Beste , et al. |
October 25, 2016 |
Tubular linear motor with magnetostrictive sensor
Abstract
A motor includes a position-sensing magnetostrictive element
that extends along a stator bore. A slider slides in the stator
bore and includes a stack of motor magnets. The stack includes a
first stack end that provides a magnetic field pattern that
magnetizes a region of the magnetostrictive element. The motor
includes shield elements such as a non-magnetic shield tube and a
magnetic flux diverter.
Inventors: |
Beste; Andre (Radevormwald,
DE), Sapp; Manfred (Kirchhundem, DE),
Walter; Ingo (Siegen, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
MTS Sensor Technologie GmbH & Co. KG |
Ludenscheid |
N/A |
DE |
|
|
Assignee: |
MTS SENSOR TECHNOLOGIE GMBH &
CO. KG (Ludenscheid, DE)
|
Family
ID: |
50979812 |
Appl.
No.: |
13/790,135 |
Filed: |
March 8, 2013 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20140252889 A1 |
Sep 11, 2014 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02K
11/21 (20160101); H02K 11/215 (20160101); H02K
11/026 (20130101); H02K 41/031 (20130101); H02K
11/012 (20200801); G01D 5/485 (20130101); H02K
1/28 (20130101); H02K 1/2733 (20130101) |
Current International
Class: |
H02K
41/02 (20060101); H02K 41/03 (20060101); G01D
5/48 (20060101); H02K 11/00 (20060101); H02K
1/27 (20060101); H02K 1/28 (20060101) |
Field of
Search: |
;310/12.01-12.33,25,23,30 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1018601 |
|
Jul 2000 |
|
EP |
|
9315378 |
|
Aug 1993 |
|
WO |
|
Other References
International Search Report and Written Opinion from the European
Patent Office mailed Sep. 4, 2014 for corresponding International
Application No. PCT/IB2014/000864, filed Mar. 6, 2014. cited by
applicant.
|
Primary Examiner: Nguyen; Hanh
Attorney, Agent or Firm: Koehler; Steven Westman, Champlin
& Koehler
Claims
What is claimed is:
1. A motor, comprising: a stator that surrounds a stator bore; a
position-sensing magnetostrictive element that extends along the
stator bore; and a slider that slides in the stator bore, the
slider having an elongated ferromagnetic portion joined to a
non-ferromagnetic portion having a stack of motor magnets operable
with the stator having a first stack end that has no stack end flux
diverter, and the first stack end provides a first stack end
magnetic field pattern that magnetizes a region of the
magnetostrictive element and that intersects the stator.
2. The motor of claim 1 wherein the slider does not include a
position indicating magnet, separate from the stack of motor
magnets, for magnetizing the region of the magnetostrictive
element.
3. The motor of claim 1 wherein the first stack end is positioned
between a second stack end and a sonic pulse sensor that couples to
the magnetostrictive element.
4. A motor, comprising: a stator that surrounds a stator bore and
that produces a changing stator magnetic field in a stator
excitation frequency range; a position-sensing magnetostrictive
element that extends along the stator bore; and a slider that
slides in the stator bore, the slider including an elongated
ferromagnetic tube joined to a non-ferromagnetic tube that
surrounds the magnetostrictive element, and a stack of motor
magnets disposed on the non-ferromagnetic tube, the stack
comprising a first stack end providing a first stack end magnetic
field that magnetizes a region of the magnetostrictive element, the
non-ferromagnetic tube shielding the magnetostrictive element from
the changing magnetic field in the excitation frequency range.
5. The motor of claim 4 wherein the non-ferromagnetic tube
attenuates the changing stator magnetic field inside the tube by at
least 3 decibels in the excitation frequency range.
6. The motor of claim 4 wherein the first stack end magnetic field
passes through the non-ferromagnetic tube.
7. The motor of claim 4 wherein the non-ferromagnetic tube
comprises stainless steel.
8. A motor, comprising: a stator that surrounds a stator bore; a
position-sensing magnetostrictive element that extends along the
stator bore; and a slider that slides in the stator bore, the
slider having an elongated ferromagnetic portion joined to a
non-ferromagnetic portion having a stack of motor magnets that
includes a first stack end and a second stack end and an end flux
diverter adjacent the second stack end, the end flux diverter
preventing a second stack end magnetic field from magnetizing the
magnetostrictive element.
9. The motor of claim 8 wherein the end flux diverter comprises mu
metal.
10. The motor of claim 8 wherein the end flux diverter has a washer
shape with a central washer opening through which the
magnetostrictive element passes.
11. The motor of claim 8 wherein the end flux diverter is in
physical contact with a magnet at the second stack end.
12. The motor of claim 8 wherein the first stack end has no stack
end flux diverter.
13. The motor of claim 8 wherein the first stack end has no stack
end flux diverter.
14. A position sensing circuit, comprising: a position-sensing
magnetostrictive element mounted at a mounting end of a motor
stator, the magnetostrictive element sensing a position along a
stator bore; a slider that slides in the stator bore, the slider
having an elongated ferromagnetic portion joined to
non-ferromagnetic portion having a stack of motor magnets, the
stack comprising a first stack end adjacent the mounting end, the
first stack end providing a first stack end magnetic field that
magnetizes a region at the position on the magnetostrictive
element; and a position-indicating circuit mounted on the mounting
end, the circuit including: a transducer that transduces a sonic
pulse adjacent the mounting end of the magnetostrictive element to
an analog pulse; a transducer circuit that receives the analog
pulse and provides a digital pulse; a measurement circuit that
provides a current pulse to the magnetostrictive element and that
measures a sonic delay time between the current pulse and the
digital pulse; and a blanking circuit couples to the transducer
circuit and that blanks a repetition of the current pulse during a
blanking time interval.
Description
BACKGROUND OF THE INVENTION
The discussion below is merely provided for general background
information and is not intended to be used as an aid in determining
the scope of the claimed subject matter.
A measurement environment with high magnetic fields is problematic
for magnetostrictive sensors. The high magnetic fields tend to
introduce noise into the measurement of position using
magnetostrictive sensors. High magnetic fields are present in
motors, especially under high load conditions, and applications of
magnetostrictive sensors in motors is difficult.
SUMMARY
This summary and the Abstract herein are provided to introduce a
selection of concepts in a simplified form that are further
described below in the Detailed Description. This Summary and the
Abstract are not intended to identify key features or essential
features of the claimed subject matter, nor are they intended to be
used as an aid in determining the scope of the claimed subject
matter. The claimed subject matter is not limited to
implementations that solve any or all disadvantages noted in the
background.
In the embodiments described below, a first end magnet of a stack
of motor magnets provides a magnetic field pattern that intersects
a magnetostrictive sensor for sensing, and also functions as a
motor magnet to produce a portion of the motor force. The
arrangement avoids the use of a flux diverter near the first end
magnet, and also avoid the use of a separate position indicating
magnet.
In the embodiments described below, a non-magnetic, electrically
conductive shield tube is disposed inside a stack of motor magnets.
The shield tube allows a magnetic field of the motor magnets to
pass through the shield, while also shielding a magnetostrictive
element from high frequency noise at motor excitation
frequencies.
In the embodiments described below, a high permability end flux
diverter is provided adjacent a second stack end of magnet stack.
The end flux diverter prevents the second stack end from
magnetizing a magnetostrictive element. In a further embodiment, an
auxiliary flux diverter in the form of a ferromagnetic pusher rod
is also provided.
In the embodiments described below, a position indicating circuit
and a motor controller are connected to a motor in order to provide
closed loop control of a motor position.
According to a first alternative aspect, a motor comprises a stator
that surrounds a stator bore; a position-sensing magnetostrictive
element that extends along the stator bore; and a slider that
slides in the stator bore and that includes a stack of motor
magnets. The slider comprises a first stack end that has no stack
end flux diverter, and the first stack end provides a first stack
end magnetic field pattern that magnetizes a region of the
magnetostrictive element and that intersects the stator.
According to a second alternative aspect, a motor comprises a
stator that surrounds a stator bore and that produces a changing
stator magnetic field in a stator excitation frequency range; a
position-sensing magnetostrictive element that extends along the
stator bore; and a slider that slides in the stator bore and that
includes a shield tube that surrounds the magnetostrictive element,
and a stack of motor magnets disposed on the shield tube, the stack
comprising a first stack end providing a first stack end magnetic
field that magnetizes a region of the magnetostrictive element, the
shield tube shielding the magnetostrictive element from the
changing magnetic field in the excitation frequency range.
According to a third alternative aspect, a motor comprises a stator
that surrounds a stator bore; a position-sensing magnetostrictive
element that extends along the stator bore; and a slider that
slides in the stator bore and that comprises a stack of motor
magnets that includes a second stack end and an end flux diverter
adjacent the second stack end, the end flux diverter preventing a
second stack end magnetic field from magnetizing the
magnetostrictive element.
In each of the first, second and third alternative aspects, the
motor can be arranged so that the slider does not include a
position indicating magnet, separate from the stack of motor
magnets, for magnetizing the region of the magnetostrictive
element
In each of the first, second and third alternative aspects, the
motor can be arranged so that the first stack end is positioned
between a second stack end and a sonic pulse sensor that couples to
the magnetostrictive element.
In each of the first, second and third alternative aspects, the
motor can be arranged so that the shield tube attenuates the
changing stator magnetic field inside the shield tube by at least 3
decibels in the excitation frequency range.
In each of the first, second and third alternative aspects, the
motor can be arranged so that the first stack end magnetic field
passes through the shield tube.
In each of the first, second and third alternative aspects, the
motor can be arranged so that the shield tube comprises a
non-ferromagnetic metal.
In each of the first, second and third alternative aspects, the
motor can be arranged so that the shield tube comprises stainless
steel.
In each of the first, second and third alternative aspects, the
motor can be arranged so that the end flux diverter comprises mu
metal.
In each of the first, second and third alternative aspects, the
motor can be arranged so that the end flux diverter has a washer
shape with a central washer opening through which the magneto
strictive element passes.
In each of the first, second and third alternative aspects, the
motor can be arranged so that the end flux diverter is in physical
contact with a magnet at the second stack end.
In each of the first, second and third alternative aspects, the
motor can be arranged to include a ferromagnetic pusher rod in
contact with the end flux diverter, the ferromagnetic pusher rod
functioning as a secondary flux diverter.
In each of the first, second and third alternative aspects, the
motor can be arranged so that the slider comprises a shield tube
that extends through the flux end diverter, and the ferromagnetic
pusher rod comprises a threaded end that is threaded onto a
threaded end of the shield tube.
In each of the first, second and third alternative aspects, the
magnets can comprise permanent magnets.
In each of the first, second and third alternative aspects, the
motor can function as a tubular linear motor to provide linear
force and linear motion between the slider and the stator.
In each of the first, second and third alternative aspects, an
inner stator sleeve can provide mechanical support for a coil stack
and provide a sliding surface to accomodate low stiction sliding of
the slider. The sliding surface can comprise PTFE plastic
resin.
In each of the first, second and third alternative aspects, the
stator can comprise resilient rings formed of compressible material
to accomodate differing rates of thermal expansion of a coil stack
and an outer stator sleeve.
In each of the first, second and third alternative aspects, the use
of a stack end flux diverter and a position indicating magnet can
be avoided, reducing cost and complexity of construction around the
first stack end.
In each of the first, second and third alternative aspects, the
magnetostrictive element can couple to transducer circuitry that
provides an output that indicates a measurement of position to a
motor controller that controls excitation currents to the coil
stack in order to provide closed loop control of the position of
the slider.
In each of the first, second and third alternative aspects, the
measurement of position can be made more accurate with the use of a
shield tube that provides noise immunity against the magnetic
fields of the motor coil during operation.
In each of the first, second and third alternative aspects, the
diversion of magnetic flux by an end flux diverter and a pusher rod
helps to avoid interference with the magnetostrictive element.
In each of the first, second and third alternative aspects, a
shield tube can alternatively be attached to a first stator end,
thereby avoiding adding a moving mass of the shield tube to the
slider.
According to one alternative, the motor can be used in conjunction
with a position sensing circuit that comprises a
position-indicating circuit mounted on the mounting end, the
circuit including:
a transducer that transduces a sonic pulse adjacent the mounting
end of the magnetostrictive element to an analog pulse; a
transducer circuit that receives the analog pulse and provides a
digital pulse; a measurement circuit that provides a current pulse
to the magnetostrictive element and that measures a sonic delay
time between the current pulse and the digital pulse; and a
blanking circuit couples to the transducer circuit and that blanks
a repetition of the current pulse during a blanking time interval.
According to another aspect, the blanking time interval is greater
than a sonic delay time interval associated with a length of the
magnetostrictive element.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A illustrates a front cross-sectional view of a tubular
linear motor with a slider shown in a first (more extended)
position and with a shield tube attached to the slider.
FIG. 1B illustrates a front cross-sectional view of the motor in
FIG. 1A with the slider shown in a second (less extended)
position.
FIG. 2 illustrates an enlarged front view of the motor in FIGS. 1A,
1B around a first stack end of the slider.
FIG. 3A illustrates a front view of an external magnetic field
pattern produced by an end magnet of the slider in FIGS. 1A,
1B.
FIG. 3B illustrates a side cross-sectional view of an external
magnetic field pattern produced by an end magnet of the slider in
FIGS. 1A, 1B.
FIG. 4 illustrates an enlarged front view of a flux diverter at a
second stack end of the slider in FIGS. 1A, 1B.
FIGS. 5A and 5B illustrate a magnetic field pattern around the flux
diverter in FIGS. 1A, 1B.
FIG. 6 illustrates a motor coupled to a transducing circuit and a
motor controller circuit.
FIG. 7 illustrates a motor that includes a shield tube that is
attached to a first stator end.
DETAILED DESCRIPTION
FIGS. 1A, 1B illustrate an exemplary embodiment of a tubular linear
motor 100 with a magnetostrictive sensor element 98. The motor 100
comprises a stator 102 that is a portion of the motor 100 that is
typically mounted to a non-moving portion of a machine (not
illustrated). The motor 100 comprises a slider 104 that is a
portion of the motor 100 that moves in a sliding fashion relative
to the stator 102. The slider 104 slides, for example, along an
axis 106 between a first (more extended) position illustrated in
FIG. 1A and a second (less extended) position illustrated in FIG.
1B. The slider 104 includes a pusher rod 108 that is typically
attached to a machine part (not illustrated) which is intended to
be moved by the motor 100 relative to the non-moving portion of the
machine. The position-sensing magnetostrictive element 98 is
mounted at a first stator end 99 of the stator 102 and extends
along a stator bore 110. The magnetostrictive element 98 is
connectable to circuitry (such as circuitry illustrated in FIG. 6)
that provides a measured position output that indicates a position
of the slider 104 relative to the stator 102. The motor 100
functions as a tubular linear motor and provides linear force and
relative linear motion between the slider 104 and the stator 102.
The magnetostrictive element 98 provides a feedback output (as
explained in more detail below in FIG. 6) that indicates the
position of the slider to a motor controller that controls the
motor 100.
The stator 102 surrounds the stator bore 110. The stator 102
comprises a coil stack 112 of electrical coils such as exemplary
coils 114, 116, 118, 120. The coils in the coil stack 112 typically
comprise toroidally-shaped coils of insulated copper wire embedded
in resin. The coil stack 112 is electrically energized (as
described in more detail below in connection with FIG. 6) to
provide a time varying magnetic field pattern inside the stator
bore 110. The stator 102 comprises an outer stator sleeve 122 that
provides mechanical support for the coil stack 112, and that also
provides a low reluctance magnetic enclosure for the coil stack
112. According to one aspect, the outer sleeve 122 comprises
magnetically soft ferromagnetic material. The stator 102 comprises
an inner stator sleeve 124 that has a tube shape. The inner stator
sleeve 124 provides mechanical support for the coil stack 112 and
provides a sliding surface to accommodate low stiction sliding
motion of the slider 104. The inner stator sleeve 124 typically
comprises PTFE plastic resin that is not magnetic.
The stator 102 comprises resilient rings 126, 128 formed of
compressible material. The rings 126, 128 are axially compressible
to accommodate differing rates of thermal expansion of the coil
stack 112 and the outer stator sleeve 122.
The stator 102 comprises retention rings 130, 132. The retention
rings 130, 132 retain the coil stack 112 in the outer stator tube
122. The retention rings 130, 132 typically comprise steel. The
retention rings 130, 132 are typically welded to the outer stator
sleeve 122.
Referring now to FIG. 1B, the slider 104 comprises sliding bearings
140, 142 that slidingly support the slider 104 on the inner stator
sleeve 124 as the slider changes position. The slider 104 comprises
a magnet stack 144 of preferably permanent magnets such as magnets
146, 148, 150, 152. According to one embodiment, the magnets in the
magnet stack 144 have a generally toroidal shape. While the magnets
in magnet stack 144 have a generally toroidal shape, these magnets
are typically magnetized in non-toroidal patterns in order to
provide a magnetic field that is external to the magnet. The slider
104 comprises a shield tube 154 that mechanically supports the
magnet stack 144. The shield tube 154 is described in more detail
below in connection with FIGS. 2, 3A, 3B.
The slider 104 comprises a first stack end 156 and an opposite
second stack end 158. As described in more detail below in
connection with FIGS. 2, 3A, 3B, the first stack end 156 has no
stack end flux diverter, and the first stack end 156 provides a
first stack end magnetic field pattern (illustrated by example in
FIGS. 3A, 3B) that magnetizes a region 160 of the magnetostrictive
element 98. The first stack end magnetic pattern (FIGS. 3A, 3B), in
addition to magnetizing the region 160, also intersects the stator
102. The region 160 moves along the magnetostrictive element 98 as
the slider 104 moves. The first stack end 156 does not include a
position indicating magnet, separate from the stack of motor
magnets, for magnetizing the region of the magnetostrictive
element. The uses of a stack end flux diverter and a position
indicating magnet at the first end 156 are avoided, reducing the
cost and complexity of construction around the first stack end
156.
The slider 104 comprises an end flux diverter 162 adjacent the
second stack end 158. The end flux diverter 162 prevents a second
stack end magnetic field from magnetizing the magnetostrictive
element 98. The end flux diverter 162 is described in more detail
below by way of an example illustrated in FIGS. 4, 5A, 5B.
As described in more detail below in connection with an example
illustrated in FIG. 6, the magnetostrictive element 98 couples to
transducer circuitry that provides an output indicating a
measurement of position to a motor controller that controls
currents to the coil stack 112 in order to provide closed loop
control of the position of the slider 104.
FIG. 2 illustrates an enlarged view around the first stack end 156
of the slider 104 of FIGS. 1A, 1B. The first stack end 156 includes
an end magnet 152. End magnet 152 is a permanent magnet and has a
generally toroidal shape. The end magnet 152 is magnetized to
produce a magnetic field (also called a magnetic field pattern)
that is external to the end magnet 152. The end magnet 152 has a
magnet end 152A that produces an external magnetic field that is
explained in more detail below by way of an example illustrated in
FIGS. 3A, 3B. The end magnet 152 is part of the slider 104 and
moves when the slider 104 moves. The end magnet 152 produces a
magnetic field that is transverse to a major axis 97 of the
magnetostrictive element 98. The end magnet 152 transversely
magnetizes a region 160 of the magnetostrictive element 98. The
region 160 moves to different positions along a length of the
magnetostrictive element 98 as the slider 104 moves. As described
in more detail below in an example in FIG. 6, the position of the
region 160 is sensed by a circuit that provides position feedback
to a motor controller. The shield tube 154 is formed of a
non-magnetic material which permits a magnetic field from the end
magnet 152 to pass through the shield tube 154 in order to
magnetize the region 160. According to one aspect, the shield tube
154 is formed of non-ferromagnetic (or very weakly ferromagnetic)
stainless steel. The shield tube 154 and the end magnet 152 are
stationary relative to one another, and the frequency of the
magnetic field of the permanent end magnet 152 relative to the
shield tube 154 is essentially zero. The magnetic field has a
near-zero frequency range and can pass through the non-magnetic
shield tube 154 with little or no attenuation. The first stack end
156 has no stack end flux diverter. The first stack end 156
provides a first stack end magnetic field pattern that magnetizes
the region 160 without the need for a separate position indicating
magnet. The end magnet 152 provides a magnetic field pattern that
performs a first function of interacting with the stator coil stack
112 to provide motor force, and that also performs a second
function of magnetizing the region 160.
FIGS. 3A, 3B illustrate portions 172, 174 of an external magnetic
field pattern produced by an end magnet 152. FIG. 3A shows a front
view comparable to the front view in FIG. 2. FIG. 3B shows a side
view that is transverse to the front view in FIG. 3A. The end
magnet 152 produces a first magnetic field pattern portion 172 that
intersects the region 160 of the magnetostrictive element 98, and
magnetizes the region 160 in a transverse direction as illustrated.
The end magnet 152 of the slider 104 produces a second magnetic
field pattern portion 174 that intersects the coil stack 112 of the
stator 102. The second magnetic field pattern portion 174 interacts
with the energized stator 102 to produce a motor force between the
slider 104 and the stator 102. The motor force moves the slider 104
relative to the stator 102. The end magnet 152 thus provides the
dual function of providing a portion of a motor force and also
magnetising the region 160. The arrangement of the end magnet 152
usefully avoids a need to provide a separate actuating magnet for
the region 160.
The shield tube 154 permits the magnetic field portion 172 to pass
through it. The shield tube 154, however, shields the region 160
from the more rapidly changing magnetic fields due to energization
of the coil stack 112. The energization of the coil stack 112
produces magnetic fields in an energization frequency range that is
higher than an essentially zero frequency of the end magnet 152. In
the higher energization frequency range, the shield tube 160 has
adequate skin effect to provide attenuation and shielding. For the
essentially zero frequency of the end magnet, however, there is no
skin effect to attenuate the field of the permanent end magnet 152.
The use of the shield tube 154 shields a portion of the
magnetostrictive element 98 from the energization field, and
reduces jitter in the measured position of the region 160. The
measurement of position is more accurate with the use of the shield
tube 154. The shield tube 154 provides immunity against the
magnetic fields of the motor coil during operation.
FIG. 4 illustrates an enlarged view of the second stack end 158 of
the slider 104 of FIGS. 1A, 1B. The second stack end 158 includes
an end flux diverter 162 that is adjacent the second stack end 158.
The end flux diverter 162 effectively prevents a second stack end
magnetic field (FIGS. 5A, 5B) from magnetizing the magnetostrictive
element 98. According to one aspect, the end flux diverter 162 is
formed of a high permeability soft magnetic material such as
mu-metal. According to another aspect, the end flux diverter 162 is
annealed in a hydrogen atmosphere to increase relative permeability
to over 50,000. According to another aspect, the end flux diverter
162 has a washer shape with a central washer opening through which
the magnetostrictive element 98 passes. According to yet another
aspect, the end flux diverter 162 is in physical contact with an
end magnet 146 at the second stack end 158. The combination of a
closed magnetic path through the washer shape, the high relative
permeability, and the physical contact between the end flux
diverter 162 and the end magnet 246 results in a magnetic circuit
that is substantially free of air gaps to provide excellent flux
diversion away from the magnetostrictive element 98.
The slider 104 includes a pusher rod 108 that is adjacent to the
second stack end 158. The pusher rod 108 is formed from a
ferromagnetic steel and is hollow. According to one aspect, the
pusher rod 108 is formed of a material with a relative permeability
in the range of a few thousand. According to one aspect, the pusher
rod 108 includes a threaded end 182 that has internal threads,
while the shield tube 154 includes a threaded end 180 that has
external threads. The threaded end 182 is threaded onto the
threaded end 180 to compress the end flux diverter 162 between the
threaded end 182 and the end magnet 146. The compression provides
for good physical contact on both sides of the end flux diverter
162 to reduce non-magnetic gaps. The threaded end 180 of the shield
tube 154 is formed of non-magnetic material, and it does not divert
magnetic flux toward the magnetostrictive element 98. The threaded
end 182 is formed of ferromagnetic material so that it diverts flux
away from the magnetostrictive element 98. The threaded end 182
serves as an auxiliary or secondary flux diverter.
FIGS. 5A, 5B illustrate portions of a magnetic field patterns 190,
192, 194 that are diverted internal to the end flux diverter 162
and the pusher rod 108 of FIG. 4. A first portion of magnetic field
flux 190 from the end magnet 146 is diverted by the end flux
diverter 162 so that the magnetic field flux 190 passes through the
end flux diverter 162 instead of through the magnetostrictive
element 98. A second portion of magnetic field flux 192 from the
end magnet 146 is diverted by the end flux diverter 162 so that the
magnetic field flux 192 passes through the end flux diverter 162
instead of through the magnetostrictive element 98. A smaller third
portion of magnetic field flux 194 from the end magnet 146 is
diverted by the pusher rod 108 so that the magnetic field flux 194
passes through the pusher rod 108 instead of through the
magnetostrictive element 98. The diversion of magnetic flux by the
end flux diverter 162 and the pusher rod 108 helps to avoid
interference with (i.e., undesired transverse magnetization of) the
magnetostrictive element 98.
As illustrated in FIG. 5A, a substantial non-magnetic gap 191
separates the end flux diverter 162 from the magnetostrictivie
element 98. The non-magnetic gap 191 comprises air and
non-ferromagnetic stainless steel. The non-magnetic gap 191
enhances diversion of flux away from the magnetostrictive element
98. The fact that the threaded end 180 of the non-magnetic shield
tube 154 passes through a center hole in the end flux diverter 162
increases the size of the non-magnetic gap 191 and improves flux
diversion.
FIG. 6 illustrates a motor 300 (similar to the motor 100 in FIGS.
1A, 1B) coupled to a transducing circuit 302 and a motor controller
circuit 304. A region 306 (similar to the region 160 in FIG. 1)
moves along a magnetostrictive element 308 as a slider 310 moves
relative to a stator 312.
The magnetostrictive element 308 extends from a first element end
314 to a second element end 316. A transducer 320 in the
transducing circuit 302 couples to a transducing region 322 of the
magnetostrictive element 308. A support circuit 321 supports the
operation of the transducer 320. The support circuit 321 receives
analog pulses from the transducer 320 and provides corresponding
digital pulses to a measurement circuit 324 and a blanking circuit
340. According to one aspect, coupling between the transducing
region 322 and the transducer 320 comprises magnetic coupling. The
measurement circuit 324 in the transducing circuit 302 is
electrically connected by insulated leads 325, 327 to the first
element end 314 and the second element end 316. The measurement
circuit 324 provides an electrical current pulse 326 that flows
along the length of the magnetostrictive element 308. The
electrical current pulse 326 magnetizes the magnetostrictive
element 308 in a generally circular direction. During the
electrical current pulse 326, the magnetization by the electrical
current pulse 326 temporarily overcome a transverse magnetization
in the region 306. At the end of the current pulse 326, the
magnetization direction of the region 306 abruptly changes from
circular to transverse. Due to the magnetostrictive effect, the
abrupt change in magnetization in the region 306 from circular to
transverse produces a sonic pulse that travels from region 306 to
the transducing region 322. The magnetostrictive element 308
functions as a sonic waveguide for the sonic pulse. The measure
circuit 324 measures a time delay T between an end of the
electrical current pulse 326 and an arrival of the sonic pulse at
the transducing region 322. The measurement circuit 324 computes a
distance D (at 328) between the region 306 and the transducing
region 322 according to a formula, distance D equals sonic velocity
V times delay T. The sonic velocity V along the magnetostrictive
element 308 is a known constant. The measurement circuit 324
provides a measured position output 330 to a measured position
input 331 of the motor controller 304. The motor controller 304
receives a position setpoint at an input 332. The motor controller
provides drive currents on bus 334 to a stack of coils 336 in the
stator. The controller compares the setpoint at input 332 to the
measured position output 330 in order to provide closed loop
control of the position of the slider 310.
The blanking circuit 340 couples to the support circuit 321 and the
measurement circuit 324 and blanks a repetition of the current
pulse during a blanking time interval. The blanking time interval
is set to ensure that one sonic pulse has dissipated before
starting another sonic pulse.
FIG. 7 illustrates a motor 700 that includes a shield tube 701 that
is attached to a first stator end 99. The slider 104 does not have
a shield tube 154 (FIG. 1) attached to it. Reference numbers used
in FIG. 7 that are the same as reference numbers used in FIGS. 1A,
1B identify the same or similar parts. Attachment of the shield
tube 701 to the first stator end 99 avoids adding a moving mass of
the shield tube 154 to the slider 104. In other respects, the
motors 100, 700 are similar.
Although the present invention has been described with reference to
preferred embodiments, workers skilled in the art will recognize
that changes may be made in form and detail without departing from
the spirit and scope of the invention.
* * * * *